1. Field of the Invention
The invention relates to an attenuator for attenuating wavelengths unequal to a used wavelength. The used wavelength is preferably a wavelength in the wavelength region of ≦100 nm, and especially preferably in the wavelength region which can be used for EUV lithography, i.e. in the region of 11 to 14 nm, especially 13.5 nm.
2. Description of the Related Art
In order to enable a further reduction in the structural widths of electronic components, and in particular in the submicron range, it is necessary to reduce the wavelengths of the light used for microlithography. It is possible to use light with wavelengths of less than 100 nm, e.g. lithography with soft X-rays, i.e. the so-called EUV lithography.
EUV lithography is one of the most promising future lithographic techniques. Currently, wavelengths in the region of 11 to 14 nm, and in particular 13.5 nm, at a numeric aperture of 0.2 to 0.3 are discussed as wavelengths for lithography. The image quality in EUV lithography is determined on the one hand by the projection lens and on the other hand by the illumination system. The illumination system should provide a uniform illumination of the field plane as far as possible in which the structure-bearing mask (i.e. the so-called reticle) is disposed. The projection lens images the field plane in a image plane (the so-called focal or wafer plane) in which a light-sensitive lens is disposed. Projection exposure systems for EUV lithography are equipped with reflective optical elements. The form of the field in the focal plane of an EUV projection exposure system is typically that of a ring field with a high aspect ratio of 2 mm (width)×22 to 26 mm (arc length). The projection systems are usually operated in scanning mode. Reference is hereby made to the following publications concerning EUV projection exposure systems:
W. Ulrich, S. Beiersdörfer, H. J. Mann, “Trends in Optical Design of Projection Lenses for UV and EUV-Lithography” in Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Publishers), Proceedings of SPIE, Vol. 4146 (2000), p. 13-24
and
M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B. Kruizinga, “Illiumination Optics Design for EUV-Lithography” in Soft X Ray and and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Publishers), Proceedings of SPIE, Vol. 4146 (2000), p. 25-34
whose scope of disclosure is hereby fully included in the present application.
In the case of illumination systems for wavelengths ≦100 nm there is the problem that the light source of such illumination systems emits radiation which can lead to an undesired exposure of the light-sensitive object in the wafer plane of the projection exposure system and moreover the optical components of the exposure system such as the multi-layer mirrors are heated thereby. In EUV systems at wavelengths of 13.5 nm for example multi-layer mirrors are used which perform a spectral filtering in the region about the EUV wavelengths, but reflect the incident radiation again with higher reflectivities from 130 nm for example. The radiation in the DUV wavelength region in particular, i.e. wavelengths in the region of 130 nm-330 nm, leads to such exposures of the light-sensitive object in the wafer plane. Radiation in the close UV region, in the visible or infrared region, i.e. wavelengths above 330 nm, lead to a heating of the mirrors.
For filtering out or attenuating this undesired radiation transmission filters made of zirconium for example are used in illumination systems for wavelengths ≦100 nm. Such filters or attenuators have the disadvantage of high losses of light. Moreover, they can easily be destroyed by heat loads.
As an alternative, it is possible to provide the filtering with grating elements according to the concept of conventional spectral filtering. In such a method, the grating period of the grating element is chosen in such a way that the radiation of the used wavelength is diffracted in the first order. With the help of a diaphragm downstream of the grating element in the beam path it is then possible to filter out especially the light of the 0th diffraction order which comprises a considerable amount of radiation with wavelengths which do not correspond to the used wavelength by blocking radiation of the 0th diffraction order. The radiation of the used wavelength of 13.5 nm for example is then diffracted substantially completely in the 1st order and allowed to pass completely to the following illumination system by the diaphragm downstream in the beam path.
The advantage of such a spectral filter is that is at least a theoretical possibility to completely surpress or block undesirable wavelengths. As a result of such an arrangement it is possible to block substantially complete the undesired DUV radiation which designates radiation in the wavelength region of 130 nm to 330 nm. Such a filter element in an EUV illumination system is shown in EP-A-1 202 291 and copending U.S. patent application US 2002/0186811 A1, whose scope of disclosure is hereby fully included in the present application.
The grating elements which are described in EP-A-1 202 291 and copending U.S. patent application Ser. No. 2002/0186811 A1 particularly provided as echelle gratings have the disadvantage that they show a total efficiency of less than 60% and place high demands on the production of the grating. As a result, the grating must have an optical functionality, for example must have optical power, so that the formation of the 1st diffraction order can occur to a certain extent in an aberration-free way at the used wavelength.
The behaviour in diffraction gratings as are known from EP-A-1 202 291 and copending U.S. patent application US 2002/0186811 A1 are described by the grating equation
                              n          ⁢                                          ⁢                      λ            p                          =                              sin            ⁢                                                  ⁢                          α              i                                -                      sin            ⁢                                                  ⁢            β                                              (        1        )            with the grating period p, the diffraction order n, the angle of incidence al relating to the surface normal of the grating, the angle β of the diffraction ray relating to the surface normal of the grating and the wavelength λ.
The grating element as described in EP-A-1 202 291 and copending U.S. patent application US 2002/0186811 Al are suitable in an illumination system for wavelengths ≦200 nm for spectral filtering in the case that the individual diffraction orders and the wavelengths are clearly separated from each other. This is achieved by a sufficiently large diffraction angle between the 0th order and a 1st order, e.g. with a diffraction angle γ=β−αi>2°. The diffraction at a wavelength of 13.5 nm for example by a larger diffraction angle γ of γ>2° for example is achieved in such a way that the grating grooves are aligned virtually perpendicular to the plane of incidence of the radiation and the grating is used under grazing incidence, i.e. the angle of incidence ax is larger than 70° relative to the surface normal of the surface. Grating periods of 500 l/mm to 1000 l/mm are thus sufficient for example. The plane of incidence is defined as the plane which is defined by the incidence vector and the normal vector of the grating surface where the incident beam pierces the grating surface. The grating vector which is situated perpendicular to the grating grooves in the tangential plane at the grating surface therefore nearly lies in the plane of incidence. If the grating vector is situated in the plane of incidence, the vector equation of the grating diffraction can be reduced to the above equation (1).
A disadvantage of the known spectral filters or attenuators is that in the case of thin films they can be destroyed by the thermal load and show only a very low efficiency in transmission. If gratings as described in EP-A-1 202 291 are used as filters or attenuators, it is possible that the DUV radiation can be blocked in particular. There is a disadvantage however that there is a very low efficiency in the region of EUV wavelengths. The maximum achievable efficiency of such gratings in the region of EUV wavelengths is only 35% to 50%.